Why is truncate DDL?

The DML versus DDL distinction isn't as clear as their names imply, so things get a bit muddy sometimes.

Oracle clearly classifies TRUNCATE as DDL in the Concepts Guide, but DELETE as DML.

The main points that put TRUNCATE in the DDL camp on Oracle, as I understand it, are:

• TRUNCATE can change storage parameters (the NEXT parameter), and those are part of the object definition - that's in the DDL camp.
• TRUNCATE does an implicit commit, and cannot be rolled back (flashback aside) - most (all?) DDL operations in Oracle do this, no DML does.

The fact that TRUNCATE doesn't run ON DELETE triggers also sets it apart from normal DML operations (but some direct path DML operations also skip triggers, so that's not a clear indicator).

That same documentation notes that DELETE generates UNDO, but TRUNCATE doesn't, so your statement is correct in this respects. (Note that TRUNCATE does generate some REDO so that the truncation can be replayed in case of restore/recovery.) But some NOLOGGING operations can also produce reduced UNDO (not sure about none at all), so that's not a clear indicator either in my opinion.

So I'd sum it up as:

• truncate is not "transactional" in the sense that it commits and can't be rolled back, and can modify object storage attributes. So it's not ordinary DML - Oracle classifies it as DDL.
• delete is an ordinary DML statement.

I think that truncate is similar to drop, alter, create as they all work on a table i.e. they all are table level commands. While 'delete' is similar to insert, select, updateas the all work on a rows i.e. they all are row level commands.